OK brakes are good, so you can count them out and go on to the next thing.
Could be something as simple as one of the outer rear wheel axle bearings going out, one of the roller cages could have broke apart and now the rollers are stacking up in the confined space of the axle and the bearing race.
You are talking a 440 power plant there, that is a lot of force on the rear end parts. Guessing it is an
8 3/4 rear end. Some of the High Powered B Bodies used the Dana rear ends cause they are a bit stronger than the 8 3/4. Next basic thing to look at after you do the heat temperature test of the outer rear end bearings after a short drive or up on the jack stands. One of those infra red temp guns work real good for this.
Then pull one of the axles out and see what that outer bearing rolls and feels like. They do get rust pitted badly over time and the bearings need to be replaced, and you were saying it sits a lot so that is not good either if there is any moisture in the rear end causing corrosion.
